Major constituents of the bacterial outer-membrane of Neisseria gonorrhoeae, including the lipooligosaccharide (LOS), pilin, and Opa proteins have been demonstrated to change expression states and antigenic nature during controlled challenge studies in human volunteers. Opa genes exhibit high frequency phase variation (on-off expression) due to changes at the DNA level resulting in translational frame-shifting. Phase variation occurs when a gene undergoes an insertion or deletion of one or more pentameric repeat units (i.e. [CTTCT], referred to as a coding repeat or 'CR') which encode the central portion of the signal-peptide. These changes are independent of homologous recombination pathways and are therefore referred to as 'non-homologous' or 'illegitimate' recombinational (IR) events. Factors influencing IR in a general manner have been determined in E. coli using opa::phoA fusions on low copy number vectors. Genes encoding the homologs of the cellular enzymes found to influence phase variation have been identified in N. gonorrhoeae, and their role in opa phase variation is presently being evaluated by deletion mutagenesis. Transcriptional influences on opa phase variation are being investigated by determining the efficiency of transcriptional initiation by each of the three different promoter types found in the opa genes of strain MS11. The role of structural elements within the repetitive region of opa genes has been extended by the demonstration that intermolecular DNA triplexes form with the addition of the pyrimidine sense repeat oligonucleotide. Interactions of Opa+ N. gonorrhoeae and E. coli with PMNs and epithelial cells have shown that expression of specific proteins leads to adherence to both of these cell types. Chimeric versions of Opa fusion proteins do not clearly identify regions of the protein which impart these biological activities. Recombinant Opa fusions have been shown to destabilize the outer-membrane of E. coli K12 strains. Strains producing E. coli B type LPS and hybrids of K12/B type LPS (e.g. HB101) are better suited to biological assays with Opa fusion proteins. The interaction of Opa+ N. gonorrhoeae with normal human sera has been partially characterized. While Opa expression does not appear to directly influence survival in human serum, the addition of human urine does appear to impart an Opa- specific protective effect (which is most pronounced in Opa+, Pili+ bacteria).